Tension leg platforms are a type of marine structure having a buoyant main body secured to a foundation on the ocean floor by a set of tethers. A typical tension leg platform is shown in FIG. 1 of the appended drawings. The point of connection between the buoyant main body and each tether is selected so that the main body is maintained at a significantly greater draft than it would assume if unrestrained. The resulting buoyant force of the main body exerts an upward load on the tethers, maintaining them in tension. The tensioned tethers substantially restrain the tension leg platform from pitch, roll and heave motion induced by waves, current and wind. It is important that the installation tension of the tethers be sufficiently great to ensure that under ordinary wind, wave and tide conditions the tethers are not permitted to go slack.
Tension leg platforms have attracted interest for use in offshore oil and gas production operations in water depths exceeding about 250 meters (820 feet). As water depths exceed 200-350 meters (656-1148 feet) the structure required to support the deck of a jacket or other conventional structure becomes extremely expensive. Tension leg platforms, however, rely on a tensile rather than compressive loading of the structure securing the platform to the ocean floor, and thus largely avoid the depth sensitivities inherent to conventional structures. It has been suggested that tension leg platforms could be employed in depths up to 3000 meters (9840 feet), whereas the deepest present application of a conventional offshore jacket is in a water depth of approximately 412 meters (1350 feet).
Though tension leg platforms avoid many problems faced by conventional platforms, they are subject to their own special problems. The most significant of these concerns buoyancy requirements. The main body of a tension leg platform must be provided with sufficient buoyancy to support not only its own weight, but also the weight of the equipment and crew facilities necessary to oil and gas drilling and producing operations. Further, the main body must also support the load imposed by the tensioned tethers. It is highly desirable to provide the tethers with buoyancy sufficient to offset some or all of their own weight. This decreases the load imposed on the main body by the tensioned tethers, eliminating the need to provide the main body with an additional degree of buoyancy sufficient to support the weight of the tethers. The decreased main body buoyancy requirements decrease the size and cost of the tension leg platform.
United Kingdom patent application No. 2,142,285A, having a priority filing date of June 28, 1983, teaches a tether design in which the tether is provided with significant inherent buoyancy. This benefit is obtained through the use of tubular tethers filled with gas pressurized to a level above the hydrostatic seawater pressure encountered at the lowest point in the tether. This use of pressurized gas prevents tether collapse in deep water applications. A system is provided for monitoring the gas pressure of the tether to detect any leaks that may occur. This design is disadvantageous in that it imposes a differential pressure across the wall of the tether which, near the ocean surface, will exceed the hydrostatic seawater pressure at the ocean floor. For an installation depth of 600 meters (1970 feet) this corresponds to a differential pressure of 6.1 megapascals (890 psi). The tether walls must be designed to withstand this high differential pressure. Also, the joints securing the individual sections of the tether together must include seals sufficient to prevent gas leakage across the great pressure differential. Further, because the tether interior forms a single, continuous channel, the entire tether could flood if a leak developed of sufficient size that air escaped more quickly than it could be replaced by the tether gas pressurization system.
As an alternative to an internal buoyancy system, buoyancy modules can be secured to the outside of submerged members. A riser buoyancy system of this type is shown in U.S. Pat. No. 4,422,801, issued on Dec. 27, 1983. This riser buoyancy system includes a number of individual air cans secured to the outer wall of the riser. Such systems would be disadantageous for use with the tethers of a tension leg platform in that they complicate inspection of the outer surface of the tether for cracks and corrosion. Also, external buoyancy systems increase the effective diameter of the tether relative to tethers having internal buoyancy systems, increasing the forces imposed on the tether by ocean currents and waves.
It would be advantageous to provide a tether buoyancy system which avoids significant pressure differentials across the wall of the tether; which maintains the outer surface of the tether free from buoyancy modules; which is controllably ballastable and deballastable to aid in tether installation and removal; which avoids the need for seals in the joints joining the individual sections of the tether; which remains substantially buoyant in the event of a leak through a tether wall; which can be deballasted continuously as individual sections of the tether are being joined in the course of tether installaion; and which accommodates a simple and reliable method for determining the location of any leak in the tether.